A general underpants type disposable diaper has front and back parts joined on both sides to thereby form joined sections on the both sides. The disposable diaper also includes an outer sheet with a waist opening and a pair of right and left leg openings, and includes an absorber that is fixed in an area of an inner surface ranging from a ventral side through a crotch portion to a back side along a central portion in the width direction. The disposable diaper can be worn by inserting both legs of a wearer through the waist opening into the leg openings.
Other than such a diaper with a single-piece outer sheet, there has been proposed a two-separated type disposable diaper which has ventral and back outer sheets (refer to Patent Document 1, for example). The two-separated disposable diaper has advantages that, at the time of manufacture, trims (unnecessary waste portions) can be reduced in punching out the leg openings, and materials for the ventral- and back-side outer sheets can be separately selected.
Meanwhile, the two-separated type can eliminate trims completely, but in this case, has no portions for covering both sides of the hip of a wearer, and thus may deteriorate in outer appearance and fit property. Therefore, the back-side outer sheet is provided with a back-side extension section having a central portion overlapping the absorber and hip cover portions extending on the both sides of the central portion, below the back-side main unit section corresponding to the joined sections in the up-down direction.
The hip cover portions in the back-side extension section may swell or curl or deteriorate in appearance. Therefore, there has been developed a diaper in which elongated resilient and elastic members such as rubber threads are fixed to the hip cover portions, in a state of being extended in the width direction at a predetermined extension ratio.    Patent Document 1: JP 2005-027839A